The tips of an aircraft airfoil, for example of a Horizontal Tail Plane (HTP), are the outer fairings at both sides, that is, the components of the HTP which is the furthest from the fuselage. FIG. 1A shows the location of the tips at an HTP, wherein it can be noted that the tips (1) complete the aerodynamic shape of the HTP leading edge.
Conventionally, tips are obtained from an aluminium alloy and are formed by several skin (2) components conformed and welded, which are internally reinforced by a structure (3) formed by metallic ribs and/or spars depending of the aircraft size, to which the tip fairing is riveted and/or welded. FIG. 1B is an example of a conventional tip structure.
The tips (1) are commonly interchangeable due to the fact that they are parts prone to accidental damage, collisions, etc.
Tips for large aircrafts have complex multi-rib structures, which can be obtained from composite materials, like Carbon Fiber Reinforced Plastic (CFRP), typically by a Resin Transfer Molding (RTM) process.
Since the tips are not Primary Structural Elements, they are generally regarded as secondary structures, and among others, must satisfy the following sizing criteria and operative requirements:    1. Stability analysis at critical aerodynamic pressure load case    2. Static strength analysis at critical loads    3. No detrimental deformation for cruise Load Case    4. Lightning strike requirements    5. Ice impact and small debris impact requirements    7. Erosion resistance requirements for the erosion prone areas of the tip    8. Interfaces analyses with torsion box, leading edge, elevator etc.
Therefore, prior art tip configurations are complex in that they are formed by several components that need to be produced separately and then assembled together, so that such as the associated manufacturing methods are time-consuming and consequently slow and expensive. Additionally, the use of metallic materials implies a significant weight penalty.
On the other hand, isogrid patterns are well-known structures that are characterized by their low weight and high rigidity. Isogrid structures are typically formed from a plate or face sheet reinforced with a framework of stringers or stiffeners interconnected at their ends, and conforming equal equilateral triangular openings. These triangular patterns are very efficient, because they provide stiffness but at the same time save material and therefore weight. The term isogrid is used because the structure acts like an isotropic material, with equal properties measured in any direction.
Patents U.S. Pat. No. 4,012,549 A, EP-0948085 (A2) describe examples of isogrid structures.